Process for producing magnetic material



1962 H. c. FIEDLER PROCESS FOR PRODUCING MAGNETIC MATERIAL Filed Sept.16, 1959 Fig.

I000 las'a run! c I 900 l-l/VAL AN/VE'AL/NG- TEMPERA Fig.2.

I IIOO C l l l .900 .950 I000 I030 FINA L A NIVEAL lA/G- 7' 5MP! RA TUAEInventor-1 Howard C.

United States Patent Ofilice 3,959,299 Patented Dec. 18, 1962 Thisinvention relates to the fabrication of polycrystalline, magneticallysoft, rolled sheet metal composed principally of an alloy of iron andsilicon having a high percentage of the grains comprising the materialoriented such that their crystal space lattices are arranged in asubstantially identical relationship to the plane of the sheet and to asingle direction in the plane of the sheet, and more particularly to animproved process for forming sheet material having this desiredorientation.

This application is a continuation-impart of application Serial No.693,043, entitled Magnetic Material, filed October 29, 1957, nowabandoned, and assigned to the same assignee as the present invention.

The sheet materials to which this invention is directed are usuallyreferred to in the art as electrical silicon steels or, more properly,silicon-iron, and are conventionally composed principally of ironalloyed with about 2.5 to 4.0 percent, preferably 2.5 to 3.5 percentsilicon and relatively minor amounts of various impurities, such assulfur, manganese, phosphorous and a very low carbon content as finishedmaterial. Such alloy-s crystallize in the body-centered cubiccrystallographic system at room temperature. As is well known, thisrefers to the symmetrical distribution or arrangement which the atomsforming the individual crystals or grains assume in such materials. Inthese materials, the smallest prism possessing the full symmetry of thecrystal is termed the unit cell and is cubic in form. This unit cube iscomposed of nine atoms, each arranged at the corners of the unit.

cube with the remaining atom positioned at the geometric center of thecube. Each unit cell in a given grain or crystal in these materials issubstantially identical in shape and orientation with every other unitcell comprising the grain.

The unit cells which are body-centered unit cubes comprising thesematerials each have a high degree of magnetic anisotropy with respect tothe crystallographic planes and directions of the unit cube, and hence,each grain or crystal comprising a plurality of such unit cells exhibitsa similar magnetic anisotropy. More particularly, crystals of thesilicon-iron alloys to which this invention is directed are known tohave their direction of easiest magnetization parallel to the unit cubeedges, their next easiest direction of magnetization perpendicular to aplane passed through diagonally-opposite parallel unit cube edges andtheir least easiest direction of magnetization perpendicular to a planepassed through a pair of diagonally-opposite atoms in a first unit cubeface, the central atom and a single atom at the unit face which isparallel to the first face. As is Well known, these crystallographicplanes and directions are conventionally identified in terms of Millerindices, a more complete description of which may be found in Structureof Metals, C. S. Barrett, McGraw-Hill Company, New York, N.Y., 2ndedition, 1952, pages 1-25, and are conventionally referred to as,respectively, the (100) plane and the corresponding [100] direction, the(110) plane and the [110] direction, and the (111) plane and the [111]direction.

It has been found that certain of the silicon-iron alloys may befabricated by unidirectional rolling and heat treatment to form sheet orstrip material composed of a plurality of crystals or grains, a majorityof which have their atoms arranged so that their crystallographic planeshave a similar or substantially identical orientation to the plane ofthe sheet or strip in a single direction in said plane. This material isusually referred to as Oriented or grain-oriented silicon-iron sheet orstrip and is characterized by having 50 percent or more of its componentgrains oriented so that four of the cube edges of the unit cells of saidgrains are substantially parallel to the plane of the sheet or the stripand to the direction in which it was rolled and a (110) crystallographicplane substantially parallel to the sheet. It will thus be seen thatthese so-oriented grains have a direction of easiest magnetization inthe plane of the sheet in the rolling direction and the next easiestdirection of magnetization in the plane of the sheet in the transverserolling direction. This is conventionally referred to as a cube-on-edgeorientation or the (110) [001] texture. In these polycrystalline sheetand strip materials, it is desirable to have as high a degree of grainorientation as is attainable in order that the magnetic properties inthe plane of the sheet in the rolling direction may approach the maximumattained in a single crystal in the direction.

In actual steel practice, these materials are prepared by casting large,thick section ingots weighing up to about 4 tons each'from alloyscontaining from about 2.5 to 4.0 percent by weight silicon, less than0.035 percent carbon, less than 0.05 percent sulfur, and less than 0.15percent manganese. By thick section, it will be understood that suchingots usually have a minimum transverse crosssectional dimension ofabout two feet. Such ingots are conventionally hot-worked into a stripor sheet-like configuration, usually less than 150 mils in thickness,commonly referred to as hot-rolled band. This band material is usuallyin an incompletely recrystallized form and may be annealed to effectcomplete recrystallization, if desired, but this is usually not done inconventional commercial practice.

The hot-rolled band is then cold-rolled with appropriate intermediateheat treatment to the finished sheet or strip thickness, usuallyinvolving at least a 40 percent 7 reduction in thickness, and given afinal or texture producing annealing treatment accompanied by adecarburizing treatment.

It has been the conventional mill practice to strip the ingot molds fromthe ingots as soon as practicable while the ingots are quite hot and toimmediately place them in a soaking pit and heat them to a minimumtemperature of about 1300" C. to 1400 C., at which temperature theyarehot-rolled,

It i a principal object of this invention to provide silicon-iron alloycastings having a fine dispersion of second phase particles which assistdevelopment of crystalline orientation by controlling grain growthduring the final anneal.

It is an additional object of this invention to provide a process fortreating silicon-iron ingots to acquire maximum [001] crystallineorientation.

Other and specifically different objects of this invention will becomeapparent to those skilled in the art from the detailed disclosure whichfollows.

In the drawings,

FIG. 1 is a graph showing the percent cube-on-edge crystal orientationdeveloped from silicon-iron cast in graphite and sand molds, as afunction of the final annealing temperature; and

FIG. 2 is a graph similar to that of FIG. 1 in which silicon-iron bodiescooled at rates of 50 C. and C. were used.

Briefly stated, the present invention is predicated upon the discoverythat there is a previously unsuspected relationship between the rate ofcooling of a silicon-iron alloy ingot containing selected amounts ofmanganese and sulfur from temperatures at which the manganese and sulfurare in solution, for example, 1300 to 1400 C. or higher and the degreeof oriented crystal texture which can be ultimately developed. Bycooling the silicon-iron ingots from temperatures on the order of 1400C. to about 800 C. at a rate of not less than about 50 C. per minute andpreferably faster, for example, at about 130 C. per minute, a finedispersion of manganese sulfide is produced which retards normal graingrowth during the final anneal and thereby enables a high degree ofcrystal orientation to be produced.

Generally, the metal can either be cooled directly from the liquidcondition, as when originally poured into an ingot mold, or can becooled from a temperature in excess of that at which the manganese andsulfur are normally in solution, this latter temperature normally beingon the order of 1300-1400 C. If the silicon-iron is cooled either fromor above these temperatures at a sufiicient rate, the manganese sulfidedispersion is properly distributed throughout the ingot and no furthertreatment need be carried out. This material is perfectly acceptable forsubsequent rolling to produce a cube-ou-edge crystal orientation. Ofcourse, if the ingot is so large as to make it difficult to attain therequired cooling rate, then it can be rolled rapidly to reduce thecross-section and thereby increase the cooling rate. It has been foundthat if the material is cooled at a rate of 50 C. per minute or faster,preferably on the order of about 130 C. per minute, then a finemanganese sulfide dispersion is obtained.

To clearly illustrate the effect of temperature on the degree ofcube-on-edge orientation which can be obtained from a material, two50-pound ingots were cast from the same heat of metal into a graphitemold in one case and into a sand mold in the other case. The purpose inusing the different types of molds was to test the effect of differentcooling rates which were obtained due to the different degrees ofthermal conductivity of the two types of molds. The composition of themetal in this instance was 3.27 percent silicon, 0.026 sulfur, 0.057manganese, 0.004 carbon, 0.009 oxygen, 0.002 nitrogen, and the remaindersubstantially all iron.

The ingot molds were 2% inches by 5 inches in crosssection and one-inchthick slabs were cut from the ingots, heated to 1000 C., rolled withoutreheating to 80 mil band, pickled, sand-blasted and heat-treated in theband stage for 5 minutes at 900 C. The band material was thencold-rolled to 25 mils in thickness, heat-treated at 860 C. for 1 to 5minutes and cold-rolled to 12 to 13 mils.

The faster is the cooling rate either from or above the solutiontemperature of the manganese sulfide, the smaller are the manganesesulfide particles and the more effective they are in preventing normalgrain growth in the final gauge strip. For example, the material whichwas cast in sand and processed to final gauge had a grain size of about0.038 millimeter when heated for minutes at 950 C., whereas the materialwhich had been cast in graphite and processed to final gauge had a grainsize of about 0.030. To ascertain the cooling rates of the metals castin graphite and sand molds, separate one-inch test slabs were heated toabove the solution temperature for the manganese and the sulfur(l3001400 C.) and then cooled at rates of 50 C. per minute and 130 C.per minute. Final gauge strip from the piece cooled at 130 C. per minutehad an average grain size of about 0.020 millimeter, while a similarstrip from the material cooled at 50 C. per minute had an average grainsize of approximately 0.028 millimeter. It will be readily noticed thatthe strip from material cooled at 50 C. per minute had a grain sizegenerally the same as that of the strip from material cast in thegraphite mold. From this data it may be concluded that the material castin the graphite had a cooling rate on the order of 50 C. per minute,

while the material cast in sand had an appreciably lower cooling rate.

The effect of the cooling rates can clearly be seen by referring to thegraphs shown in FIGS, 1 and 2 of the drawings where the amount ofcube-on-edge texture obtained after two hours at the temperatureindicated is shown. The degree of cube texture developed in the materialcast in graphite can be seen by referring to the curve designated by thenumeral 10. In this case, textures as high as about percent wereobtained when the material wa heat-treated within the range of fromabout 925 to 975 C. On the other hand, the material processed from theingot which was cast in the sand, and therefore had a cooling rate ofless than 50 C. per minute, had a maximum texture of only on the orderof 40 percent when heated at from 925 to 975 C.

Referring to the two curves shown in FIG. 2 of the drawings, the curve15 represents strip from that body which was cooled at a rate of 130 C.per minute, while the numeral 16 indicates strip from that materialwhich was cooled at about 50 C. per minute. It will be noted that thetextures obtained from the material cooled at 50 C. per minutecorrespond substantially with that indicated by curve 10 in FIG. 1.Here, once again, the maximum texture obtained was on the order of 80percent and the texture obtained dropped off rapidly as higher finalannealing temperatures were used. This feature of decreasing orientationwith increasing annealing temperatures is an important one since itpermits the use of only comparatively low heating rates in thetexturedeveloping anneal. The percent cube-on-edge texture developed isshown in Table I as a function of the heating rate during the finalanneal.

Table I Percent TextureI-Ieating Rate Slab (gooling Rate One difficultyinvolved is the fact that the texture takes a substantially longer timeto develop at 950 C. than it does at some higher temperature, forexample, 1050 to 1100 C. In this connection, the material which wascooled at 130 C. per minute developed textures approaching percent andtextures in excess of 80 percent, even when the annealing temperaturewas raised to 1100 C. Thus, if the cooling rate is kept at asubstantially high level to precipitate a fine, grain-boundary pinningsecond phase, i.e., the manganese sulfide or other inclusion, then highorientation can be readily and easily developed through use of thehigher temperatures. It has been observed that annealing times on theorder of one-half hour are needed to develop adequate texture attemperatures of about 950 0, whereas complete texture development canoccur in as little time as 5 minutes when the annealing temperature israised to 1050 to 1100 C. or possibly slightly higher. It is thusobvious that while the cooling rate of 50 C. per minute will permit theattainment of good cubc-on-edge textures, the higher cooling rates, forexample, the C. per minute, permit even further advantages to berealized.

All of the textures were measured by the conventional torquemagnetometer test described later in the specification.

Additional examples illustrating the invention are set forth in thefollowing representative examples. In these examples, a number of heatsor alloys of silicon-iron having comparable compositions as shown inTable II were prepared by melting commercial electrolytic ironcontaining less than 0.01 percent manganese by spectrographic analysis,with appropriate amounts of silicon as com- 'r'nrcial, low aluminum, 98percent 'ferrosilicon, sulfur as iron sulfide, carbon as an iron-carbonalloy made from electrolytic iron and graphite, and titanium in the formof titanium sponge. With regard to the titanium addition, it will beappreciated that it is desirable that the sulfur not be present in thesealloys as-cast as iron sulfide in order to obtain optimum rollingcharacteristics. In usual commercial practice, manganese is used forthis purpose, forming manganese sulfide; however, titanium may equallywell be used, forming titanium sulfide. Obviously, manganese andtitanium may be used or, in fact, other addition elements known to formstable sulfides may also be used.

All of the representative heats weighed 50 pounds and were made in aninduction furnace with magnesia cruc1- bles. The alloys were poured attemperatures between 16001650 C. into various sized molds made ofdifferent materials. For example, heats 1, 4 and 5 were cast into castiron molds having a regular mold cavity having a rectangular transversecross-section with a minimum transverse width of 3%" and a minimumthickness of 1%" and a length of 18". Heats 2 and 3 were cast intographite molds having a rectangular transverse crosssection with aminimum of transverse width of 6%.", a minimum thickness of 3 A and alength of 9". Heats 6 and 7 were cast into cast iron molds having aregular mold cavity having a square transverse cross-section with aminimum transverse width and thickness of 3% and a length of 14". Theingots having the 1%" thickness will cool substantially faster than theingots of 3%" thickness.

The ingots were cast into the molds, permitted to cool, removed from themolds and subjected to the following fabrication procedure.

Ingots from heats 1, 4 and 5 were machined to form 1%" thick by 3%" wideslabs, heated to 1000 C. 1n a dry hydrogen (dewpoint about 60 F.)atmosphere and hot-rolled to 80 mil thick hand. These hot-rolled bandswere then annealed at 900 C. for /2 hour at that ternperature in anatmosphere of dry hydrogen. It should be noted that any conventionalprotective atmosphere or a vacuum may be used in place of hydrogen atthis point. After cooling, the annealed bands were cold-rolled to milthickness strip. These strips were then subjected to an intermediateanneal in a dry hydrogen atmosphere at about 860 C. Again, otherprotective atmospheres or vacuum may be used. The strips were in theheated zone for about 3%. minutes and at the 860 C. temperature forabout one minute. After cooling, the grain sizes of these anneal stripswere determined and found to range from an average measured diameter of0.017 to 0.025 millimeter. The annealed strips were then cold-rolled to12 mil thick strip and decarburized by heating for 5 minutes .at 800 C.in wet hydrogen. The decarburized strips were then placed in'a metalretort in a furnace and dry hydrogen (90 dew point) was flowed throughthe retort as temperature was increased at a rate of 100 C. per hour to1200 0, held at that temperature for 8 hours and cooled at about 100 C.per hour to 600 C., then furnace-cooled to about 300 C., at which pointthe retort was removed from the furnace.

The ingots from heats 2 and 3 were processed as follows: A slice ofabout 1" thick by 6%" wide by 9 long was cut from each ingot, theresulting slabs heated to 1000 C. and hot-rolled to mil thick hot-rolledbands. The bands were annealed in the same manner as the bands fromheats 1, 4 and 5 and cold-rolled to 25 mil thick strip. One portion ofthe strip from heat 2 was then annealed in dry hydrogen under the sameconditions set forth for the intermediate anneal given strips from heats1, 4 and S and another portion from heat 2 and all of strip from heat 3were annealed under the same conditions except the temperature wasraised to 950 C. The intermediate grain size of the 860 C. anneal stripfrom heat 2 had an average measured grain size of 0.007 millimeter,while the 950 C. annealed portion of this same material had a grain sizeof 0.021 millimeter. The intermediate grain size of the strip from heat3 was 0.025 millimeter. Both strips from heat 2 and the strip from heat3 were then cold-reduced to 12 mils thickness and given thedecarburization and final annealing treatment given the strips fromheats 1, 4 and 5.

The ingots from heats 6 and 7 were heated to 1000 C. in dry hydrogen andforged .to a rectangular transverse cross-section of 1%" by 4". Aportion of each forged bar was then reheated to 1.000 C. in dry hydrogenand rolled to 80 mil thicknesshot-rolled hand. These bands were annealedas set forth previously and cold-reduced to 25 mil intermediatethickness strip and heated in the same manner in similar strips fromingots l, 4 and 5 at 860 C. The intermediate grain size from the stripfrom heat 6 was found to be 0.027 millimeter and from the strip fromheat 7 to be 0.016 millimeter.

Conventional torque magnetometer test specimens, oneinch diameter disks,were prepared from each finished strip and were rotated, as is wellknown, in a unidirectional magnetic field of 1000 oersteds. The degreeof (110) [001] preferred orientation or texture expressed as percenttexture was determined by torque measurements for each sample and isreproduced in Table III. This value is calculated from the torque valuesfound for the specimens based on the value of 150,000 ergs/ cubiccentimeter for percent texture as found in single crystals of analogouscomposition having the specified orientation. Additionally, the finalcarbon content and final sulfur content was determined for thesematerials.

Table III Intermediate Grain Percent Final Final Ingot Size (mm.)Texture Carbon Sulfur (Percent) (Percent) Under certain circumstances,heavier gauge finished sheet or strip-like material may be desired. Forexample, if 25 mil thick sheet having a high degree of orientation isdesired, the previously disclosed hot-rolled 80 mil band should beannealed to effect complete recrystallization. This may be accomplishedby annealing the band at about 900 C. for a period of time necessary toeffect complete recrystallization. The recrystallized band may be thencold-reduced to the final thickness, e.g., 25 mils, without intermediateheat treatment, decarburized and subjected to the same final annealingtreatment as set forth with respect to the 12 mil strips previouslydisclosed. As an example of the effectiveness of this treatment toproduce strong textures, samples of the hot-rolled band from heat 1 werecold-rolled to the 25 mil thickness with and with- V Table IV PercentPercent; Texture Texture (Band not (Band Annealed) Annealed) It will beseen, therefore, that strong textures are developed in this heaviersheet or strip material only if the hot-rolled band is recrystallizedbefore cold-working.

From the material set forth in detail in the present specification, itmay be seen that the cooling rate of the as-cast structure through thetemperature range where the sulfide first precipitates and through thetemperature range immediately below that temperature range where growthof these particles may occur is critical to the degree of preferredgrain orientation which may be achieved in the final strip material.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A process for forming a fine dispersion of sulfide inclusions insilicon-iron castings which are adapted to be formed into sheet-likebodies having a majority of their constituent grains oriented in the(110) [001] texture comprising cooling said casting from a temperatureat which the sulfides are in solution to about 1000 C. at a rate of atleast 130 C. per minute, reducing said casting by rolling to form asheet-like body, and annealing said sheet-like body to develop saidgrain orientation therein.

2. A process for the fabrication of sheet-like bodies of polycrystallinesilicon-iron alloy having a majority of their constituent grainsoriented in the (110) [001] texture comprising the steps of casting analloy consisting essentially of from about 2.5 to 4.0 percent silicon,less than about 0.04 percent carbon, from about 0.010 to 0.05 percentsulfur and the remainder substantially all iron, said sulfur beingpresent as sulfide inclusions in the form of a fine dispersion in saidcasting at temperatures below about 1300 C., cooling the casting at arate of at least 130 C. per minute from a temperature 8 at which thesulfides are in solution to about 1000" C., producing a metal sheet fromsaid casting, and annealing said metal sheet to refine the same and todevelop grain orientation therein.

3. A process for the fabrication of sheet-like bodies of polycrystallinesilicon-iron alloy having more than a majority of their constituentgrains oriented in the [001] texture comprising the steps of melting analloy consisting essentially of from about 2.5 to 4.0 percent silicon,less than 0.035 percent carbon, from about 0.015 to 0.05 percent sulfur,and the remainder substantially all iron, casting said alloy into a moldto produce a slab-like ingot, said sulfur being present as sulfideinclusions in the form of a fine dispersion in said castings attemperatures below about 1400 C., cooling said ingot at a rate of atleast C. per minute to a temperature between room temperature and lessthan 1000 C., removing said ingot from said mold, heating said ingot toabout 1000 C., rolling said heated ingot to reduce said minimumtransverse dimension to form an elongated sheet-like body less than milsin thickness, cold-rolling said elongated body to effect at least a 40percent reduction in thickness and annealing said cold-worked body in asuitable reducing atmosphere to cause the desired recrystallized textureto develop and to purify the strip.

4. A process as defined in claim 3 in which said hotreduced elongatedsheet-like body is subjected to a heat treatment to cause the completerecrystallization of the body prior to cold reduction.

5. A process as defined in claim 3 in which said hotreduced elongatedsheet-like body is cold-rolled to effect at least a 40 percent reductionin thickness, heated in a protective atmosphere to cause the cold-Workedmetal to recrystallize, and cold-rolled to eifect at least a 40 percentreduction in thickness prior to the decarburization andtexture-development heat treatments.

References Cited in the file of this patent UNITED STATES PATENTS2,534,141 Morrill et al. Dec. 12, 1950 2,618,843 Goodsell Nov. 25, 19522,867,558 May Jan. 6, 1959 OTHER REFERENCES

1. A PROCESS FOR FORMING A FINE DISPERSION OF SULFIDE INCLUSIONS INSILICON-IRON CASTINGS WHICH ARE ADAPTED TO BE FORMED INTO SHEET-LIKEBODIES HAVING A MAJORITY OF THEIR CONSTITUENT GRAINS ORIENTED IN THE(110)(001) TEXTURE COMPRISING COOLING SAID CASTING FROM A TEMPERTURE ATWHICH THE SULFIDES ARE IN SOLUTION TO ABOUT 1000*C. AT A RATE OF ATLEAST 30*C. PER MINUTE, REDUCING SAID CASTING BY ROLLING FOFM ASHEET-LIKE BODY, AND ANNEALING